Transport across membranes I, II: Ion Gradients, Electrochemical Driving Forces, Membrane Potential Flashcards
molecules/ions can only cross via
Transporters or channels that act as gates
where are transport systems located?
both the cell membrane and on the membrane of intracellular organelles
what electrically active cells depends on constant changes in ion fluxes? (action potential)
neurons, cardiomyocytes, and muscle cells
simple diffusion occurs through the ____________
lipid bilayer
describe diffusion through a Semi-Permeable Barrier
transport can be classified into which two categories?
active transport or passive transport (diffusion)
active transport can be classified into which two categories?
primary and secondary
secondary active transport happens via which two methods?
symports (same direction) and antiports (opposite directions)
diffusion can be classified into which two categories?
simple and facilitated
what type of transport does bulk flow derive from?
diffusion
simple diffusion happens via which two methods?
via bilayer and via ion channels
bulk flow is driven via which two methods?
hydrostatic pressure (force exerted by a fluid) and osmotic pressure (pressure required to prevent the flow)
What are the 3 modes of passive through the plasma membrane according to Fick’s First Law
- simple diffusion via lipid bilayer
- simple diffusion via ion channels
- facilitated diffusion with aid of carrier proteins
Is energy required for passive transport through the cell membrane?
No energy is required for passive transport.
what mode of transport does the cell invest energy to force ions/molecules to cross the membrane?
active transport
describe Brownian movement in diffusion via lipid bilayer
dictates chance spread from high concentration to the low concentration area, creating a homogenized, equalized average concentration & driven by thermal energy
what does a semi-permeable barrier consider changes in?
net flux of molecules across membrane as the concentration gradient changes
True or False: The two sides of the
membrane are parts of one continuous compartment
True
True of False: The compound randomly diffuses down its concentration gradient from low concentration to high concentration.
False.
from high concentration to low concentration
what is the driving force in diffusion through a semi-permeable barrier?
the concentration gradient
What happens to the net flux of molecules when there is a large initial concentration gradient?
A large initial concentration gradient creates a significant net flux of molecules from the side with higher concentration to the side with lower concentration.
How does the net flux change as the concentration gradient decreases?
As the concentration gradient decreases, the net flux of molecules also decreases.
What is the concentration on side 1 and side 2 during the intermediate stage of diffusion?
During the intermediate stage, the concentration on side 1 has decreased to 0.75 M, while the concentration on side 2 has increased to 0.25 M.
What happens to the net flux during the intermediate stage of diffusion?
The net flux decreases as the concentration gradient between the two sides decreases
What is the final state of concentrations on both sides of the membrane?
In the final state, the concentrations on both sides of the membrane become equal and the net flux becomes 0. This ensures a dynamic steady state
what determines how permeable the membrane is for a solute and how readily it diffuses across the bilayer?
the solubility of the solute in lipid
if the solute can cross, where does it go down?
its concentration gradient
define concentration gradient?
A concentration gradient is the difference in the concentration of a substance between two regions. It represents the gradual change in the concentration of solutes in a solution as a function of distance through a solution.
what does the permeability of the bilayer to different molecules depend on?
hydrophilic/hydrophobic nature
True or False: the hydrophobic molecule penetrates easily
True
How are hydration shells created?
Polar molecules associate with water molecules
The smaller the molecules the _____ polar it is and the more rapidly it diffuses across the bilayer.
the smaller the molecule, the LESS polar
lipids without proteins are….
highly impermeable to ions, no matter how small they are
Give examples of Hydrophobic molecules
O2, CO2, N2, steroid hormones (testosterone, progesterone, estrogen)
Give example of small uncharged polar molecules
H20, urea, glycerol, NH3
Give examples of large uncharged polar molecules
glucose, sucrose
give examples of Ions
H+, Na+, HCO3-, K+, Ca2+, CI-, Mg2+
if lipid solubility is high, does it create a strong or weak concentration
gradient across the
membrane?
strong concentration
If the lipid solubility is high, does the molecule’s transfer accelerate or decelerate?
If lipid solubility is
high, the membrane,
accelerating the
molecule’s transfer.
what does Fick’s Law describe?
passive movement molecules down its concentration gradient
Fick’s Law Equation
Flux (J) (molecules per unit time) =
(C2 – C1) · (Area · Permeability coefficient) / Thickness
what is the permeability coefficient ?
the mobility of the solute in the diffusion path
What are the C2 and C1 in Fick’s Law ?
C2 is the higher concentration and C1
is the lower concentration
- C2 - C1 = Concentration difference.
what is the thickness in Fick’s Law?
the length of the diffusion path
Simple diffusion through ion channels as well as facilitated diffusion with the aid of carrier proteins are mediated by what?
*Hint: the answer in in the question
proteins that function as gates
True or False: All modes of transport follow Fick’s Law of Diffusion.
False.
Not all modes of transport follow Fick’s Law of Diffusion. Fick’s Law specifically applies to passive diffusion, where the movement of molecules is driven by a concentration gradient and does not require energy input.
is diffusion via carrier proteins passive?
Yes, but solute transport is accelerated/
facilitated by the presence of
the carrier protein (compared
to unaided diffusion through
the membrane).
What does the protein provide in both channels and carriers?
a permeation path
How does diffusion through a channel/transporter facilitate the transport?
by making it less difficult energetically
in simple diffusion, is peeling off the hydration shell lowly or highly energetic?
peeling off the hydration shell is highly energetic
is the activation energy for delta G in diffusion through the bilayer low or high?
the activation
energy ΔG for diffusion through the bilayer is high
True or False: A transporter protein reduces the delta G for transmembrane diffusion of the solute
True.
How does a transporter protein reduce the delta G for transmembrane diffusion of the solute ?
by making the peel-off easier
- forming non-covalent
interactions with the dehydrated
solute to temporarily replace the
hydrogen bonding with water
- providing a hydrophilic
transmembrane passageway.
describe facilitative transport
molecule binds to protein transporter and the mechanism/driving force is passive diffusion
- momentary binding to protein transporter that makes transport saturable
the molecule (substrate) is released to the other side due to ______ concentration
lower
define carrier
a transmembrane protein that binds substrates on one side of a membrane and transports them to the other side
- spontaneous conformational change
define spontaneous conformational change
natural change in 3D shape of substrate/molecule
- possesses 1+ binding sites specific for molecules of certain substances
what happens when velocity of transport reaches maximum rate transport
it gets saturated
how does simple diffusion differ from Facilitative transport
- it does not require binding to a protein
- the graph remains linear
- no saturation
in facilitated diffusion, saturation kinetics is due to …..
a finite # of carriers & the conformation changes
in terms or kinetics, channels are ________, while carriers are ________.
channels are linear where carriers are saturable
what causes saturation?
more solutes being transported than the transporters can handle
- maxing-out of the increasing rate to a steady state rate (Vmax)
do channels have a weak or strong interaction with the permeating compound? and is the permeation slow or fast?
weak interaction with permeating compound/molecule/substrate
- very rapid permeation
- no conformational changes
- no saturation (simple diffusion)
what are ionophores used for?
increasing permeability of membrane to specific ion
what are ionophores?
small hydrophobic molecules
what classes can ionophores be divided into?
- mobile ion carriers
- Valinomycin –> K+
- FCCP –> H+
- A23187 –> Ca2+/Mg2
- synthetic channel formers
- Gramicidin A)
Sometimes a “cracked” transporter in facilitated diffusion can be used as a _______ in simple diffusion.
channel
how do transporters get “cracked” in the first place?
they get stuck in a confirmation where a crack opens in the protein complex allowing selected ions to flow through
- so it works more as a channel
give examples of “cracked” transporters
some glutamate transporters
- CFTR
in terms of simple diffusion, it is driven by a _______ gradient
concentration
in terms of Facilitated Diffusion and Ion Transport, they are driven by an ___________ gradient
electrochemical
diffusion can ________ electrical gradient
generate
What is a difference in electrical charge between 2 spots called?
Electrical Potential/ Voltage
How do we measure electrical gradients?
in Volts
What is Ohm’s Law?
Speed of flow (current) of electrical charge depends on the electrical concentration gradient (electrical potential) & properties of the flow path (resistance).
What is Ohm’s Law?
I = V/R
V = I x R
I = current (amp)
V = electrical potential (volt)
R = resistance (R, Ohm , horse shoe looking ahh symbol)
define membrane potential
net charge inside the cell - net charge outside the cell
in most cases, the inside of the cell is slightly more ________.
negative
The difference in charges establishes an _________________________.
electrical gradient
When the chemical concentration is higher inside than outside, the direction of the chemical driving force is ____________.
outward
when the chemical concentration is higher outside than inside, the chemical driving force is ____________.
inward
in both cases, if permeable the molecules will move passively in the direction of the direction of the driving or…..
down the concentration gradient
charged ions have what kind of force
electrical driving force
on a positively charged ion, the direction of the electrical driving force is
inward
the positive ion will go in,
down its electrical gradient
the direction of the electrical driving force on a negatively charged ion is the _________ that of the positive ion.
opposite
The direction of the electrical gradient depends both on
concentration and on the charge
Do the two driving forces, chemical and electrical gradients, work dependently or independently?
independently
